1. Field of the Invention
This invention is related to assemblies, and more particularly to assemblies that have a back plate that can be attached to a circuit board.
2. Description of the Related Art
In many applications it is advantageous to attach a back plate to a circuit board. FIG. 1 is an exploded view of circuit board assembly 8. Assembly 8 has back plate 10 that attaches to circuit board 12. Back plate 10 has a plurality of openings. Some of the openings, such as opening 18, go through a portion of back plate 10 to accommodate components. Other openings, such as openings 20, 22, 24, 26, 28, run through the entire back plate 10, and are typically for fasteners. The fasteners can be used to fasten back plate 10 to circuit board 12 and to the equipment or device that receives assembly 8. Back plate 10 is typically fabricated out of metal and is preferably thick enough to be rigid.
Typically, back plate 10 is fabricated out of inexpensive lightweight metal, such as aluminum. The length, width, and thickness of back plate 10 are large enough to receive circuit board 12 and keep the circuit board fairly rigid when it is attached to the back plate. The length and width of back plate 10 are typically about the same as the length and width of circuit board 12. Top side 16 of back 10 plate fits on one side 28 of circuit board 12. Circuit board 12 is thin, and therefore difficult to work with because it bends and warps. The additional thickness provided when circuit board 12 is attached to back plate 10 makes circuit board 12 fairly rigid, preventing circuit board 12 from bending or warping. It is much more accurate and economical to install components with automatic surface mount equipment. It easier to install components onto circuit board 12 when it is rigid because if the circuit board surface is warped the circuit board may not be able to go through the automatic surface mount equipment due to its shape, and when it can go through the equipment, it is difficult to deposit components in the right locations due the circuit board continuing to bend as component are deposited. It is also easier to install circuit board 12 into equipment when it does not bend or warp since it is easier to machine flat surfaces to receive circuit board 12 in the equipment than to machine surfaces that can receive a warped or bent circuit board.
Circuit board 12 has one or more layers 30, 32, 34, 36, 38. Some of layers, such as layers 30, 34, 38 have a conductor, such as copper, deposited on one or on both sides to form a particular pattern of tracks that is the printed wiring. These layers 30, 34, 38 are formed out of plastic or out of a plastic and glass compound. Other layers, such as layers 32, 36 are formed of an epoxy material and hold the layers of circuit board 12 together. Tracks are used to provide a power grid, a ground grid, and other electrical connection for components 56, 58. For example, in circuit board 12 designed for both digital components 58 and RF components 56: layer 34 has tracks that are the connections for digital component 58 on the top side of layer 34, and tracks that are the digital ground on the bottom side of layer 34. Circuit board 12 also has layer 30 with tracks that are the connections for RF components 56 on the top side of layer 30, and tracks that serve as RF ground on the bottom side of layer 30. Circuit board 12 has openings 44, 4648, 50, 52 that run through all of the layers. These openings are for the fasteners that connect circuit board 12 and back plate 10.
Circuit board 12 designed to work with RF component 56 has connect openings 54, 55 located near the location of the RF components. Connect openings 54, 55 are through layer 30 that has the tracks are the RF ground. There is a connection through connect openings 54, 55 between RF component 56 are the RF ground, this connection provides an electrical conduction path between RF component 56 and the RF ground.
Circuit board 12 has component openings 40, 42 through the entire depth of circuit board 12. Components 56, 58 fit through component openings 40, 42, respectively. A thin metal layer, hereinafter flange 60, 62, at the bottom of components 56, 58, contacts back plate 10. Components 56, 58 are either integrated circuits or discrete devices connected to circuit board 12 by leads 64, 66, and 68, 70, respectively. Leads 64, 66, 68, 70 are attached to the printed wiring on top surface 72 of circuit board 12.
As described above, typically back plate 10 is fabricated out of inexpensive lightweight metal, such as aluminum. One problem with fabricating back plate 10 out of aluminum is that the thermal expansion coefficient of aluminum is very different from the thermal expansion coefficient of the metal that forms flanges 60, 62. This makes it difficult to solder components 56, 58 to back plate 10. The temperature of flanges 60, 62 and back plate 10 are raised during the soldering process. As flanges 60, 62 and back plate 10 cool after they are soldered together, flanges 60, 62 contract at a different rate than back plate 10, thus pulling apart the solder and breaking the connection. This can cause components 56, 58 to break away from the rest of assembly 8.
Fabricating back plate 10 out of a material such as a copper, that has a thermal expansion coefficient that is close to the thermal expansion coefficient of flanges 60, 62 creates a different problem. Since back plate 10 should be large enough to support circuit board 12 and to provide a good thermal conduction path away from circuit board 12. Fabricating a large back plate from a metal that is a good conductor would tremendously increase the cost of the assembly, going against the long term industry goals and trends of reducing cost. Furthermore, because copper is a heavy material, a large copper back plate would make the assembly heavy and therefore more difficult to support. Additionally, the current trend is to combine more and more components on a single circuit board, requiring larger circuit boards and therefore larger back plates, which further increases the cost and weight of the back plate.
Another problem with assembly 8 is that the electrical connection between RF component 56 and the RF ground is through connect openings 54, 55. The size of connect openings 54, 55 is restricted by the area that can be apportioned on leads 64, 66. Typically, connect openings 54, 55 are small. Because connect openings 54, 55 are small it is difficult to ensure that a good connection is made between component 56 and ground.